DENVER (AP) — One by one, the Native American runners walked by the simple marble gravestone of one of two U.S. Army officers who refused to fire on their ancestors in one of the worst atrocities in the settlement of the American West.

The Cheyenne and Arapaho runners touched the stone in Denver's oldest cemetery Wednesday after trekking from the scene of the Sand Creek massacre on the plains about 180 miles (290 kilometers) away. Their journey marked the 150th anniversary of the attack.

Some of the tribal members and others who joined them added to the rocks on top of the marker for Capt. Silas Soule. Next to the headstone stood a framed photo of the Kansas grave of Lt. Joseph Cramer, the other officer who refused to participate in the massacre.

Led by people holding tribal flags and staffs bearing eagle heads, about 70 runners then made their way through the industrial neighborhood to downtown and the state Capitol. There, Gov. John Hickenlooper apologized for the massacre on behalf of the state.

Soule and Cramer witnessed but ordered their men not to participate in Col. John Chivington's Nov. 29, 1864, attack on Cheyenne and Arapaho camped along a dry creek bed. About 200 tribal members, many of them women and children, were killed.

The raid came after months of fighting and rising tensions between Indians and white settlers. Many of the newcomers, like Cramer and Soule, were lured by the promise of striking it rich in gold, which pushed the tribes from their traditional lands.

The Indians believed they were safe after making peace with the commander of nearby Fort Lyon. They even flew a U.S. flag at the camp.

Both Soule and Cramer attended peace conferences with the Arapaho and Cheyenne chiefs earlier in the year and heard U.S. military commanders tell them they were in no danger.

Some descendants of Sand Creek survivors credit Soule and Cramer for their own existence, believing further bloodshed might have wiped out their ancestors too.

Many also credit Soule's graphic written account of the massacre with helping push leaders to start coming to terms with the massacre. A letter he wrote resurfaced in 2000 as Congress considered creating a Sand Creek historic site.